Previous studies from this laboratory have shown a defect in celluar immunity of burned mice with a decreased in vitro cytotoxicity of lymphocytes. Investigations into the mechanism of this phenomenon indicate a defect in the induction of immunity of spleen T cells. Other experiments were performed to study the host defense mechanism of burned animals, which depends upon the migration of lymphocytes and macrophages to the burned area in order to aid in the process of phagocytosis of foreign antigens, especially bacteria. Our results indicate that migration of macrophages and lymphocytes is significantly decreased after burn trauma. This effect is not due to abnormal production of lymphokines that influence migration, since macrophage and lymphocyte inhibition factor production was normal in lymphocytes from burned mice. In addition, previous experiments showed a decrease of extravascular fluid movement in mice with age. Studies were made of the effect of varying concentrations of NaCl administered in the diet on the effects upon fluid movement and, in the long run, upon longevity of mice.